The invention relates to a method for uniquely aligning a patient's visual line of sight (LOS) to the optical axis of an ophthalmic apparatus that is used for laser surgery or diagnostic measurements. In particular, the invention is useful for surgical laser procedures on the anterior segment of the eye, including but not limited to corneal refractive surgery, and especially wherein such procedures follow a prescription based on prior corneal topographic measurements, thus requiring unique cross-correlation of the patient's LOS between different machines. Other important uses of the invention include, for example, applications involving laser interventions on the retina, especially where correlation with images generated independently by, e.g., a fundus camera, are of advantage.
Ophthalmic surgery using laser beams to modify, for example, the corneal surface or to treat conditions such as glaucoma (by penetrating structures such as the iris or sclera) is a relatively recent development, but has met with substantial success. The precision of a laser intervention typically far exceeds that of mechanical interventions, such as an intervention with a scalpel.
Exploiting the precision of a laser intervention to its fullest requires at least equal precision in locating and tracking critical structures of the eye. In an eye-tracking application involving a sighted eye, there are three functions typically provided to reduce errors in positioning the eye. First, the error in axial location of the eye is measured with respect to a reference location on the apparatus, and the apparatus is accordingly adjusted to reduce this error to a minimum. Second, decentration of the eye with respect to the optical axis of the apparatus is determined, and the apparatus is adjusted to reduce this error to a minimum. Third, the error in angular orientation of the eye's line of sight with respect to a reference direction associated with the apparatus is determined, and the individual is instructed to reduce this error to a minimum by visually fixating on a target.
As an example, many new corneal refractive surgical techniques require the line of sight of the patient's eye to be precisely aligned with the optical axis of the surgical laser instrument. In general, corneal refractive surgery in ophthalmology, including treatment of myopia, hyperopia, and astigmatism, requires, for best results, precise mapping of the topography of the corneal surface so as to establish reference curvatures and/or elevations against which subsequent surface-modifying treatments can be selected and measured. However, to be useful, it is very important to center such topographic measurements upon an axis of the eye that is related to the eye's actual functions and yet can be registered experimentally to the machine axis in an unambiguous manner. The usual method of determining the topography of the cornea is to have the patient look at a small source of light (i.e., a fixation point) while simultaneously placing the patient's head into a "correct" position. This "correct" position is frequently determined by an operator observing the location of reflected light from two projected beams. These two alignment beams can either overlap at a predetermined point along the optical axis of the laser surgical apparatus (i.e. so-called parallax ranging, as used in the Computed Anatomy videokeratography machine and described in U.S. Pat. No. 4,863,260), or the beams may include cross hairs that are projected onto the limbus at the equatorial plane (as used in the EyeSys topography machine).
In conjunction with the fixation point, these alignment beams, in theory, uniquely define the optical axis of the ophthalmic tracking and surgical system and also position the eye the correct distance from the optics of the instrument. However, as will be discussed further below, neither of the above-mentioned approaches using alignment beams is satisfactory for ensuring that measurements have been taken within a reproducible reference frame, i.e., a reference frame that can be relied upon to treat the patient on his or her line of sight (LOS).
The importance of using the LOS as one axis of the reference frame lies in two facts. First, critical vision is centered on the LOS of the eye, irrespective of the direction in which the (mechanical) axis of symmetry of the eye is pointed. Second, the LOS is the only metric of the eye which can be defined without ambiguity (See D. Duke-Elder and D. Abrams, "Ophthalmic Optics and Refraction", Vol. V, In System of Ophthalmology, editor D. Duke-Elder, C. V. Mosby Co., St. Louis, 1970, pp. 134-138). By definition, the LOS (which is sometimes referred to as the "principal line of vision") is the chief ray of the bundle of rays passing through the pupil and reaching the fovea, thus connecting the fovea with the fixation point through the center of the entrance pupil. Thus, the line of sight is defined by the patient, which means by the employment of the patient's complete vision system, not by external measurements of the eye, e.g., corneal apex, iris, sclera, retina, limbus, etc., as would be required for determining the axis of symmetry (or the so-called optical axis of the eye).
In particular, it is acknowledged that for best optical performance, it is the intersection of the LOS with the cornea that marks the desired center for the optical zone of refractive surgical procedures, i.e., resulting in the largest zone of glare-free vision.
It should also be noted that the emphasis made herein on clarifying what is meant by the LOS is due to the preponderance in the literature of references to the visual axis, a term which has no clinical significance. In reality the visual axis, which is defined as the hypothetical line connecting the fixation point with the fovea and passing through the nodal points, cannot be experimentally located with any accuracy since the eye is not a centered optical system (the fovea is not located on the optical axis). In fact, it is by definition the LOS which constitutes the most accurate representation of the visual axis in the sense of being amenable to measurement.
Several techniques for aligning the patient's line of sight involve directing the patient to focus on a single fixation point. However, definition of any line requires two conditions. When the patient views a single fixation point, the alignment of the patient to the optical axis of the instrument is dependent upon the ability of the operator to judge, in the parallax ranging method, the overlapping of two beams at the corneal apex or, in the cross hair method, the alignment of two cross hairs upon the limbus. In either case, there is no patient interaction with the machine, other than following directions as to which direction to look or move.
The efficacy of both of the techniques mentioned above is thus seriously compromised by the fact that their alignment is a combination of two separate alignments: translational/angular displacement with respect to the optical axis in a plane perpendicular to the optical axis, and alignment of the eye in distance (focussing) along the instruments optical axis. These two alignments are inextricably intertwined, making a simple focussing action very difficult, if not impossible, to achieve without repeated realignment in the plane perpendicular to the optical axis.
For example, Bille et al., in U.S. Pat. No. 4,848,340, teaches a method of locating the (erroneously labeled) visual axis by directing visible light towards the patient's eye on which he or she can fixate. According to Bille, alignment to the optical axis is considered achieved by this fixation. The fixation aspect of this patent does not, however, specify the apparent distance to the visual target. Furthermore, with a single collimated fixation beam entering the eye (i.e. the target appearing to be at infinity), the observer would have no clue as to errors in transverse alignment, the extent of which corresponds at least to the finite size of the fovea. As such, the methods described in said U.S. Pat. No. 4,848,340 cannot ensure coaxial, much less collinear, alignment of the line of sight to the optical axis of the apparatus because looking at a single point of light (even if at infinity) does not result in a unique (i.e. translation invariant) solution for the LOS.
In view of the above, it is an object of the present invention to provide a method for uniquely aligning the LOS of a patient's eye with the optical axis of an ophthalmological laser surgery or diagnostic instrument, thus providing a means for explicitly ensuring collinearity of the eye's and the machine's chief rays.